In general, in an image forming apparatus using an electrophotographic system, a toner is conveyed via a conveyance medium including an electrostatic latent image carrier such as a photoconductor and an intermediate transfer medium such as a transfer belt and deposited at a desired position on a transfer medium such as paper. The toner is then subjected to contact bonding by heat rollers or the like and fixed onto the transfer medium, thereby forming an image on the transfer medium.
In a two-component developing agent, a toner is mixed with a carrier which is a magnetic fine particle whose surface is coated with an epoxy resin, etc. and used. The carrier is stirred together with the toner in a developing machine, thereby charging the toner and depositing it onto a photoconductor, etc. due to an electrostatic effect.
In recent years, in an image forming apparatus which is free from a system for exchanging a developing agent, in order to reduce the exchange frequency, it is required to realize a long life of a developing agent. In realizing a long life of a developing agent, a problem to be caused due to a stirring stress between a toner and a carrier in a developing machine is visualized. As the problem to be caused due to a stirring stress, deterioration of a carrier due to peeling of a surface coating layer of the carrier and separation or burying of an inorganic oxide to be externally added in a toner matrix particle are exemplified.
As a method for suppressing peeling of a surface coating layer of a carrier, a method for thickening the surface coating layer is exemplified. In order to achieve thickening, it may be considered to perform a coating treatment by combining plural kinds of surface treating agents. However, there are involved problems such as an increase of costs, complication of treatment steps and heterogeneity of the surface coating layer. Furthermore, even by subjecting to a thickening treatment, the initial carrier surface resistance cannot be kept. By repeating a printing durability test, the surface coating layer is peeled away. This is because the surface resistance is lowered, thereby causing failures such as toner flying and fog.
As a method for suppressing the separation of an external additive of a toner, an enhancement of an adhesive force of an external additive to a core toner is exemplified. The adhesive force can be increased depending upon an external addition condition such as a time and a revolution number at the external addition. However, when the external additive is deposited strongly too much, the external additive is buried in the core toner, thereby affecting flow properties or charge properties.
As described previously, in order to suppressing the deterioration of a carrier or the separation of an external additive of a toner, various investigations have been made. However, it is the present situation that such problems have not been solved yet.